In this specification, where a document, act or item of knowledge is referred to or discussed, this reference or discussion is not an admission that the document, act or item of knowledge or any combination thereof was at the priority date, publicly available, known to the public, part of common general knowledge; or known to be relevant to an attempt to solve any problem with which this specification is concerned.
Whilst the following discussion relates to coalbed methane, a person skilled in the art will understand that the invention is not limited to coalbed methane and can be used in the recovery or injection of other gases and/or liquids, including other hydrocarbons such as oil in shale and unconventional hydrocarbon resources.
Coalbed methane (CBM) (also known as coalbed gas, coal mine methane, and coal seam methane) is a form of natural gas extracted from coal beds. The term refers to methane adsorbed into the solid matrix of the coal. The presence of this gas is well known from its occurrence in underground coal mining, where it presents a serious safety risk due to its explosive nature. Coalbed methane is distinct from a typical sandstone or other conventional gas reservoir, as the methane is stored within the coal by a process called adsorption.
To extract the gas, a steel-encased hole is drilled into the coal seam (eg 100-1500 meters below ground). The hole exposes a face of the coal seam to lower pressure as opposed to the compressive pressure naturally applied to the rest of the seam which induces gas and water to escape from the coal seam. Additionally, water may be pumped from the coal seam which again induces the liberation of gas. The gas is collected and sent to a compressor station and, in turn, into natural gas pipelines.
Geologically, water typically permeates a coal seam and water pressure holds in place any CBM present. Producing CBM requires first removing the water to decrease the pressure on the coal matrix, allowing free gas to flow into the well bore. The ‘produced water’ is either reinjected into isolated formations in the reverse manner, released into streams, used for irrigation, or sent to evaporation ponds. The water typically contains dissolved solids such as sodium bicarbonate and chloride.
The methane desorption process follows a curve (of gas content vs. reservoir pressure) called a Langmuir isotherm. The isotherm can be analytically described by a maximum gas content (at infinite pressure), and the pressure at which half that gas exists within the coal. These parameters (called the Langmuir volume and Langmuir pressure, respectively) are properties of the coal, and vary widely. A coal in one state and a coal in another state may have radically different Langmuir parameters, despite otherwise similar coal properties.
As production occurs from a coal reservoir, the changes in pressure are believed to cause changes in the porosity and permeability of the coal. This is commonly known as matrix shrinkage/swelling. As the gas is desorbed, the pressure exerted by the gas inside the pores decreases, causing them to shrink in size and restricting further gas flow through the coal. As the pores shrink, the overall matrix shrinks as well, which may eventually increase the space the gas can travel through (the cleats), increasing gas flow.
The potential of a particular coalbed as a CBM source depends on the following criteria. Cleat density/intensity: cleats are joints confined within coal sheets. They provide permeability to the coal seam. A high cleat density is required for profitable exploitation of CBM. Also important is the maceral composition: maceral is a microscopic, homogeneous, petrographic entity of a corresponding sedimentary rock. A high vitrinite composition is ideal for CBM extraction, while inertinite hampers the same.
The rank of coal has also been linked to CBM content: a vitrinite reflectance of 0.8-1.5% has been found to imply higher productivity of the coalbed.
The gas composition must also be considered, because natural gas appliances are designed for gas with a heating value of about 1000 BTU (British thermal units) per cubic foot, or nearly pure methane. If the gas contains more than a few percent non-flammable gasses such as nitrogen or carbon dioxide, it will have to be blended with higher-BTU gas to achieve pipeline quality. If the methane composition of the coalbed gas is less than 92%, it may not be commercially marketable for gas sale, but at 50% or less may be used for power generation.
The current practice of drilling a bore into a coal seam to extract CBM raises a number of practical issues. One of the key problems is that the coal seam is often soft and collapses on itself making it difficult to bore. In fact, drilling operations are generally more difficult in coal per se, and in soft coal usually impossible to drill any distance as a result of jamming by such collapsing material.
There thus exists a need for an alternative method for recovering gas and/or liquids, such as CBM and other hydrocarbons, from gas and/or liquid reservoirs, especially soft geological materials such as coal, shale or sand.